This invention takes as its starting point PCT application number PCT/GB00/04416 (SCIENTIFIC GENERICS LIMITED) filed 20 Nov. 2000 and entitled ‘Safety Device’. The closest prior art known to the applicant is this PCT application and any art acknowledged or on record for its patent prosecution since filing.
FIGS. 5a–5g in PCT application number PCT/GB00/04416 and their associated text explain that formed on top of each arm, 541, 542 towards the back inside edge of each, is a first (540b) and a second (540a) half of a priming button 540, which, when pushed forwards by a user, causes the back halves 541a, 542a of the locking arms to pivot outwardly about the hinge regions 541d, 542d. 
This PCT-published priming button arrangement 540 etc has been superseded by the present invention which not only works much better but on quite different principles. These differences, and their inherently inventive nature when compared with this most relevant prior art document, will become apparent as this present specification unfolds.
Other trigger mechanisms exist in this field and can be broadly summarised as:                Those using single sided spring actions to return to the safe position, these have a tendency for the end caps to tilt towards the needle axis in use and have the potential to snag the needle tip preventing use;        Those using coiled/compressed spring actions to return to the safe position; here the number of parts in the assembly precludes the use of modem single-piece moulding techniques, incurring undesirable additional manufacturing costs.        
An objective of the present invention is to provide an improved safety device incorporating a trigger mechanism that includes a priming function which guards against accidental so-called “needle stick” when not in use and enables the needle to be unsheathed during use in a controlled manner and then automatically resheathed to ensure safe further contact.